1. Field of the Invention
The invention is directed to a double resonance antenna arrangement for a nuclear magnetic resonance apparatus having at least one conductor section that defines an examination region.
2. Description of the Prior Art
Nuclear magnetic resonance apparatuses are utilized for imaging as well as for spectroscopy in medical diagnostics. Different operating frequencies are thereby required for the excitation of the atomic nuclei for imaging than for spectroscopy.
The operating frequency given a basic magnetic field of 1.5 T in imaging thus amounts to 64 Mhz, whereas a frequency of 26 Mhz is required for spectroscopy of, for example, phosphorous. Such large differences in the operating frequencies require antenna arrangements having corresponding resonant frequencies. It would be expedient to employ an antenna arrangement that is set or can be set to both operating frequencies.
There are various proposals for a realization of antenna arrangements having two operating frequencies. U.S. Pat. No. 5,038,105 discloses an antenna arrangement wherein a first terminal is provided for a first operating frequency and a second terminal is provided for a second operating frequency. This antenna arrangement thus exhibits different resonant behavior dependent on the terminal via which a transmission signal is supplied or via which a reception signal is taken. A disadvantage of this arrangement is that two terminals are provided, and thus two connecting lines to the antenna arrangement must be laid.
In the antenna arrangement disclosed in European Application 0 256 370, the different operating frequencies are generated via tuning elements controlled from the exterior. Although a common terminal is provided in this known arrangement for all frequencies; however, additional control lines for switching tuning elements are required.
PCT publication WO 90/12 328 discloses a double-tuned resonant "bird cage" antenna arrangement. Bird cage antenna arrangements differ from other antenna arrangements in that a discrete phase distribution must be observed around the circumference of the coil from 0.degree. through 360.degree.. The antenna arrangement disclosed therein has a single terminal for two operating frequencies and has double resonance without switching elements. This is achieved because the individual elements that form the bird cage antenna arrangement behave as a low-pass filter at low frequencies and behave as a high-pass filter at high frequencies. This can be achieved in a band-pass bird cage antenna arrangement as well as in a band rejection bird cage arrangement by adding reactances. A disadvantage of such a bird cage antenna arrangement is that a sufficient uniform antenna characteristic can only be achieved for one of the two operating frequencies.